The present invention relates to a method for the preparation of a catalyst carrier, to a hydrocarbon desulfurization catalyst prepared using the carrier, and to a process for hydrodesulfurizing a hydrocarbon feedstock using the aforementioned catalyst. More particularly, it relates to a method for the preparation of a porous catalyst substantially free of macropores (those having diameters above 1000 .ANG.), and containing at least one metal and/or metal compound of Groups VIB and VIII of the elements. Still more particularly, it relates to a catalyst which comprises a predominantly alumina carrier component which is substantially free of macropores, has a particular micropore size distribution, and contains the aforementioned metal and/or metal compounds. It also relates to a hydrocarbon hydrodesulfurization process using the catalyst.
The pressing need for desulfurizing hydrocarbon oils obtained from petroleum processing is well known. When these stocks are combusted as a fuel in the usual manner, the sulfur present in the hydrocarbon becomes a serious pollutant of the atmosphere in the form of sulfur oxide gases.
Typical operating conditions for hydrodesulfurization processes include a reaction zone temperature of 600.degree. F. to 900.degree. F., a pressure of 200 to 3000 psig, a hydrogen feed rate of 500 to 15000 SCF per barrel of oil feed, and a catalyst such as nickel or cobalt and molybdenum or tungsten on a porous refractory support.
A problem which has been recognized in the case of hydrodesulfurization of heavy oils is that if the heavy oil contains organometallic compounds, the effective catalyst activity tends to decline relatively rapidly, particularly when the impurity is more than about 10 to 20 ppm metals such as dissolved nickel and vanadium. These metallic impurities are said to deposit on the surface and in the pores of the hydrodesulfurization catalyst.
An approach to this problem of metals impurity deactivation of hydrodesulfurization catalyst has been to alter the pore structure of the catalyst. However, the answer as to what pore structure is best has not been easily obtained, and in fact there remains a conflict in the answer suggested by the prior art. U.S. Pat. Nos. 4,066,574; 4,113,661; and 4,341,625, hereinafter referred to as Tamm '574, Tamm '661, and Tamm '625, the contents of which are incorporated herein by reference as if fully set forth in ipsis verbis, have discussed the conflict in the art and suggested a solution.
Tamm's patents disclose that heavy oil feedstocks containing metals, particularly residuum feedstocks, are hydrodesulfurized using a catalyst prepared by impregnating Group VIB and Group VIII metals or metal compounds into a support comprising alumina wherein the support has at least 70% of its pore volume in pores having a diameter between 80 and 150 .ANG.. An especially outstanding hydrodesulfurization catalyst, in terms of very low deactivation rate, is attained by using an alumina support of the above pore size distribution.
In Tamm '661 the catalyst is prepared by taking a predominantly alpha-alumina monohydrate, sized in the range below 500 microns, and treating it with a particular amount of monobasic acid. The acid and the resulting mixture is then at least partially neutralized by admixing with an aqueous solution of a nitrogen base such as aqueous ammonia. The solution contains 0.6 to 1.2 equivalents of base per equivalent of acid. The treated and neutralized product is converted into a catalyst carrier by shaping as desired, drying, and calcining. Finally, the catalyst support is impregnated with the aforementioned metals.
It would be advantageous if the catalysts and processes of Tamm '574, '661 and '625 could be improved by imparting greater catalytic activity to the catalyst over a longer period of time.